Microsystem process networks

ABSTRACT

Various aspects and applications of microsystem process networks are described. The design of many types of microsystems can be improved by ortho-cascading mass, heat, or other unit process operations. Microsystems having exergetically efficient microchannel heat exchangers are also described. Detailed descriptions of numerous design features in microcomponent systems are also provided.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/588,999, filed Jun. 6, 2000, which is incoroporated herein as ifreproduced in full below.

This invention was made with Government support under ContractDE-AC0676RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to microchannel devices and particularlymicrochannel devices that are capable of unit process operationsinvolving the transfer of mass or heat.

BACKGROUND OF THE INVENTION

Systems involving heat or mass transfer are crucial to ourindustrialized society. Examples of such systems include: powergeneration, chemical processing systems, and heating and coolingsystems. For more than 100 years, scientists and engineers haveendeavored to increase the efficiency or reduce the cost of thesesystems.

Battelle, Pacific Northwest National Laboratories and others have beenusing microtechnology to develop Microsystems for carrying out processesthat had previously been conducted using far larger equipment. Thesesystems, which contain features of 1 millimeter (mm) or less, maypotentially change heat and mass transfer processing in ways analogousto the changes that miniaturization have brought to computing.Microsystems can be advantageously used in small scale operations, suchas in vehicles. Microsystems that can be economically mass-produced canbe connected together to accomplish large scale operations.

The production of hydrogen from hydrocarbon fuels, for use in fuelcells, are one example of an application that has been proposed formicrosystems. Fuel cells are electrochemical devices that convert fuelenergy directly to electrical energy. For example, in a process known assteam reforming, a microsystem can convert a hydrocarbon fuel (or analcohol such as methanol or ethanol) to hydrogen and carbon monoxide.The hydrogen is fed to a fuel cell that reacts the hydrogen and oxygen(from the air) to produce water and an electric current. The CO could,in a reaction known as the water gas shift reaction, be reacted withwater to produce additional hydrogen and carbon dioxide. Thus, fuelcells offer many potential advantages over conventional internalcombustion engines—fuel cells can be more energy efficient and they donot produce nitrogen oxides and ozone that are the primary unhealthfulcomponents of smog.

Despite long and intensive efforts, there remains a need for energyefficient and cost effective systems for carrying out operationsinvolving heat or mass transfer. There is also a need for compactsystems or microcomponent systems for conducting processes that areconventionally carried out on a larger scale. This patent describes newsolutions for more efficient and cost-effective systems utilizingmicrocomponent technology.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a microcomponentapparatus for conducting unit operations comprising a microcomponentdevice having a first inlet, first exit, a first array of microchannels,and a second array of microchannels. During operation, a stream entersthe first inlet of the microcomponent device and is distributed amongthe first array of microchannels and a first unit operation is performedon the stream. The stream exits through the first exit and exits themicrocomponent device. A processing device is connected to the firstexit of the microcomponent device. The processing device is capable ofmodifying the stream by a second unit operation. An outlet of theprocessing device is connected to a second inlet of the microcomponentdevice through a second inlet and the second array of microchannels isconnected to the second inlet and a second exit is connected to thesecond array of microchannels. During operation, the stream re-entersthe microcomponent device and is distributed among the second array ofmicrochannels where a third unit operation is performed on the stream,and the stream exits through the second exit and exits themicrocomponent device. In a preferred embodiment, the third unitoperation is the same as the first unit operation—this is an example ofortho-cascading. The invention also includes methods that use themicrocomponent apparatus in the manner described.

In another aspect, the invention provides a microchannel devicecomprising: a first zone, wherein during operation, at least one unitprocess operation is performed, and that, during operation, functions ata first temperature; a second zone that, during operation, functions ata second temperature; wherein the first temperature is different thanthe second temperature; and

a microchannel heat exchanger that is disposed between the first zoneand the second zone. During operation, a stream flows from the secondzone through the microchannel heat exchanger to the first zone andsubsequently flows back through the microchannel heat exchanger to thesecond zone. Also, during operation, within the microchannel heatexchanger, heat is exchanged between the stream flowing from the secondzone to the first zone and the stream flowing from the first zone to thesecond zone; and the heat exchanger has a thermal power density of atleast 0.6 W per cubic centimeter and an exergetic efficiency of at least80%. The invention also includes methods that use the microchanneldevice in the manner described. In a preferred embodiment, the heatexchanger in the above-described microchannel device has an exergeticefficiency of at least 80% (preferably 85 to 95%) when the first zone isat a temperature of 600° C. and the second zone has a temperature of200° C.—this does not mean that the apparatus is defined to operate atthese temperatures, rather these temperatures provide a precisetemperature for testing apparatus for exergetic efficiency.

In another aspect, the invention provides a microstructure architecturecomprising at least two layers: a first layer comprising a continuousflow microchannel, and a second layer adjacent the first layer thatcomprises at least one microchannel. The first layer and the secondlayer cooperate to form at least two unit operations, and the flowmicrochannel forms at least a portion of the at least two unitoperations. Preferably, the flow microchannel is substantially straight.In one preferred embodiment, the two unit process operations are heatinga gas and vaporizing a liquid. In another preferred emodiment, the flowmicrochannel forms at least a portion of at least three unit operations.

In a further aspect, the invention provides microchannel apparatus inwhich the microchannel walls have gaps that allow pressure to equalizeamong the microchannels. In another aspect, the invention provides amicrocomponent apparatus that contains a catalyst chamber in which thereare upper and lower flow paths separated by a space, and upper and lowercatalyst supports. The upper catalyst support is disposed substantiallyin the upper flow path, and the lower catalyst support is disposedsubstantially in the lower flow path.

In yet another aspect, the invention provides a microchannel apparatuscomprising a header, at least two flow microchannels, and at least twoorifices. Each orifice connects the header with each flow microchannel.The ratio of the cross-sectional area of each orifice to thecross-sectional area of the flow microchannels connected to saidorifices is between 0.0005 and 0.1, more preferably between 0.001 and0.05. This apparatus is especially useful as a water vaporizer.

In still another aspect, the invention provides a method of exchangingheat in a microchannel device, in which a first stream in a microchannelexchanges heat with a second stream, wherein the first stream remains inthe microchannel and, subsequently, the first stream exchanges heat witha third stream without leaving the microchannel.

In a further aspect, the invention provides a method of conducting unitoperations in microcomponent apparatus comprising: performing a firstunit operation on a first stream in a first microcomponent cell,subsequent to the first unit operation, performing a second, discreteunit operation on the first stream to make a modified stream, then in asecond microcomponent cell, performing the first unit operation on themodified stream, to accomplish a single unit operation for the samepurpose as the a first unit operation on the first stream. In apreferred embodiment, the first stream is a heat exchange fluid and thefirst unit operation in a microcomponent cell is heat exchange, withheat being transferred from the first stream to provide heat for anendothermic process; where the second unit operation modifying firststream comprises reheating the first fluid by reheating from a heatsource or by adding additional fuel or oxygen and performing combustionreactions. Examples of endothermic processes include drying, boiling,evaporation, endothermic chemical reactions, and desorption. In anotherpreferred embodiment, heat is transferred for an exothermic process,such as an exothermic chemical reaction, or a sorption process such as agas into a liquid or a gas onto a solid. In another preferredembodiment, the first unit operation comprises a chemical reaction, suchas steam reforming, and the second unit process comprises mass transfer,such as adding additional hydrocarbon reactant to first stream.

In another aspect, the invention provides a microcomponent device forconducting unit operations comprising: a first microcomponent devicehaving a first inlet, first exit, first header, a first array ofmicrochannels and a second array of microchannels. The first inlet isconnected to the first array of microchannels. The first array ofmicrochannels are connected to a first exit that is connected to thefirst header. During operation, a first stream enters the first inlet ofthe first microcomponent device and is distributed among the first arrayof microchannels and a first unit operation is performed on the firststream, the first stream then passes through the first exit into thefirst header. The first header being is capable of modifying the firststream by a second unit operation. The first header is connected to asecond array of microchannels within the first device. During operation,the first stream enters the first microcomponent device and isdistributed among the second array of microchannels wherein the firstunit operation is again performed on the first stream.

The invention further provides a method of transforming exergy in amicrocomponent device in which a portion of the chemical exergy of afirst stream is converted to physical exergy and a portion of thisphysical exergy is transferred to chemical exergy in a second stream.The first stream and the second stream do not mix. The step oftransferring at least a portion of said physical exergy to chemicalexergy in a second stream has a an exergetic efficiency of at least 50%;and the step of converting at least a portion of the first chemicalexergy to physical exergy has a thermal power density of at least 0.6watts per cubuc centimeter.

The invention also provides a method of designing a microcomponentapparatus, comprising using exergetic analysis to analyze a systemhaving microcomponents that are involved in at least one unit processoperation; and designing a change in the microcomponent apparatus basedon that exergetic analysis. The exergy analysis can be used to identifythe causes and calculate the magnitude of exergy losses. The inventionalso provides a chemical process system comprising: a fuel cell; a heatpump; and a chemical conversion unit capable of producing fuel for thefuel cell. During operation, the fuel cell produces heat at a firsttemperature, and the heat pump increases the temperature of the heat toa higher temperature. Heat from the heat pump is transferred to thechemical conversion unit. Preferably, the heat pump contains acompressor. While the invention recognizes that there are numerousapproaches to exergetic analysis, “exergetic efficiency” of a specifiedpercent refers to the exergetic efficiency as calculated by the specificmethod described herein.

The invention, in various aspects and embodiments can provide numerousadvantages including: reduced exergy destruction, higher powerdensities, process intensification, improved exergetic efficiency,reduced costs of construction and operation, new abilities to performoperations in small volumes, relatively high flow rates and reducedtemperatures, reductions in size, and improvements in durability.

The subject matter of the present invention is distinctly claimed in theconcluding portion of this specification. However, both the organizationand method of operation, together with further advantages and objectsthereof, may further be understood by reference to the followingdescription taken in connection with accompanying drawings wherein likereference characters refer to like elements.

GLOSSARY OF TERMS

A “cell” refers to a separate component, or a volume within anintegrated device, in which at least one unit operation is performed. Inpreferred embodiments, the cell has a width less than about 20 cm,length less than about 20 cm, and height less than about 3 cm.

“Flow microchannel” refers to a microchannel through which a fluid flowsduring normal operation of an apparatus.

“Microchannel” refers to a channel having at least one dimension that isabout 2 mm or less, preferably 1 mm or less. The length of amicrochannel is defined as the furthest direction a fluid could flow,during normal operation, before hitting a wall. The width and depth areperpendicular to length, and to each other, and, in the illustratedembodiments, width is measured in the plane of a shim or layer.

“Microcomponent” is a component that, during operation, is part of aunit process operation and has a dimension that is 2 mm or less,preferably 1 mm or less.

“Microcomponent cell” is a cell within a device wherein the cellcontains micro components.

“Ortho-cascading” refers to a process in which first unit operation isperformed on a first stream in a first microcomponent cell, subsequentto the first unit operation, a second, discrete unit operation isperformed on the first stream to make a modified stream, then in asecond cell, the first unit operation is again performed on the modifiedstream, to accomplish a single unit operation. The first unit operationsin the first and second cells has the same purpose.

“Unit process operation” refers to an operation in which the chemical orphysical properties of a fluid stream are modified. Unit processoperations (also called unit operations) may include modifications in afluid stream's temperature, pressure or composition. Typical unitprocess operations include pumping, compressing, expanding, valving,mixing, heating, cooling, reacting, and separating.

“Thermal power density” refers to the heat transfer rate divided byvolume of the device, where volume of the device is the sum of streamvolume involved in heat transfer and the walls between the streams,calculated in the portion of the device where there is a significantamount of heat transfer (thus excluding long stretches of piping, etc.).

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent with color drawings will be provided by theOffice upon request and payment of the necessary fee.

FIG. 1 illustrates a hot gas reinjection scheme for reforminghydrocarbons.

FIG. 2 illustrates representative increases in overall system efficiencyof a fuel cell power generation system operated using reformate producedutilizing hot gas reinjection.

FIG. 3 is a schematic diagram illustrating integration of steam reformerreactors and hydrogen separation membranes.

FIG. 4 is a flow diagram for a compact steam reforming unit with fuelcell.

FIG. 5 is a flow diagram for a compact steam reforming unit with solidoxide fuel cell.

FIG. 6 is a top down view of a shim for a microchannel heat exchanger.

FIG. 7 is a graph of observed versus predicted effectiveness for theheat exchanger.

FIG. 8 is a graph of energy and exergy efficiency as a function ofheating rate for a microchannel heat exchanger.

FIG. 9 is a graph of exergy destruction as a function of heating ratefor a microchannel heat exchanger.

FIG. 10 is a color drawing of a compact steam reforming unit.

FIG. 11 a is a top down view of a heat exchanger shim in a microchannelrecuperator.

FIG. 11 b is a top down view of a shim in a microchannel recuperator.

FIG. 12 a is a top down view of a shim of a microchannel heat exchanger.

FIG. 12 b is a top down view of a shim of a microchannel heat exchanger.

FIG. 13 a is a top down view of a shim of a microchannel heat exchanger.

FIG. 13 b is a top down view of a shim of a microchannel heat exchanger.

FIG. 14 a is a top down view of a vaporizer shim in a microchannelrecuperator.

FIG. 14 b is a bottom up view of a vaporizer shim in a microchannelrecuperator.

FIG. 14 c is a top down view of a heat exchanger shim in a microchannelrecuperator.

FIG. 14 d is a bottom up view of a heat exchanger shim in a microchannelrecuperator.

FIG. 15 a is a top down view of a reaction chamber layer shim in areactor.

FIG. 15 b is a blow up of a reaction chamber showing dimensions in mils.

FIG. 15 c is a top down view of a spacer shim in the reaction chamberlayer.

FIG. 15 d is a top down view of a shim in the heat exchanger layer of areactor.

FIG. 15 e is a top down view of an endplate of a reactor.

FIG. 16 is a diagram of exergy balance for a compact steam reformingunit.

FIG. 17 is a diagram of enthalpy balance for a compact steam reformingunit.

FIG. 18 is a black and white rendition of the compact steam reformingunit shown in FIG. 10.

DETAILED DESCRIPTION

The invention involves a core of closely related concepts that areperhaps most easily understood by dividing the descriptions into thethree aspects of: ortho-cascading, exergetic efficiency, and systemconstruction. In many embodiments, these are closely related concepts.For example, ortho-cascading can contribute to exergetic efficiency, andvarious preferred embodiments of system constructions can beortho-cascaded, exergetically efficient, or both. Some preferredembodiments of each of these aspects are discussed below.

Ortho-Cascading

Conventional process flowsheet development involves identifying asequence of unit operations, such as chemical reactors, separations, andheat exchangers, which are interconnected by process streams to form aprocess train. At the most simplistic level, raw materials are fed tothe process at one end, the material passes through a network of unitoperations, and products come out the other end. This inherenttwo-dimensional structure gives rise to the terms ‘up-stream’ and‘down-stream’ in reference to processes. Superimposed on this inherenttwo-dimensional structure are concepts of recycle and multiple stages.Recycle streams flow opposite to the general direction of material flowthrough the system; they can flow from a down-stream process to anup-stream process or back to the inlet of the generating process.Multiple stages generally refers to cases where a single unit operationis accomplished in a sequence of steps, such as a series of continuousstirred tank reactors (CSTR) or a train of mixer-settlers in solventextraction.

Conventional flowsheet development is driven largely by economies ofscale. There may be rare cases where an engineer would choose to divideup a unit operation into a sequence of parallel vessels or trains, suchas adding parallel trains in a capacity expansion. Dividing up a unitoperation into a sequence of parallel trains substantially increases thecost of conventional process technology. This is in marked contrast tomicrotechnology where the smallest building block is a microchannel,which are assembled into arrays that form cells. Practical constraints,desired operational characteristics, and flow distribution issuestypically limit the size of a single cell. Consequently, multiple cellscan be included in a device with multiple devices in parallel trains toachieve the necessary throughput.

This invention recognizes and takes advantage of the inherent modularityof the microtechnology in process network development. The modularityallows a system designer to mentally replace a single icon representinga single unit operation on a flowsheet with a stack of iconsrepresenting parallel cells or devices, giving the flowsheet a threedimensional quality. Conceptually, the third dimension gives uniqueopportunities for ortho-cascaded networks, which are in essencecombinations of parallel, serial and cross-current processing withinarrays of the microchannel architecture building blocks. This conceptcan be extended to unlimited combinations of co-current,counter-current, and crosscurrent processing for high efficiency heattransfer, mass transfer, and/or chemical reactions. As an example, thishigh degree of versatility coupled with the high efficiency and higheffectiveness achieved in microchannel heat exchangers, can give rise tohigh fidelity heat exchange networks that minimize irreversible losses.

This three-dimensional quality of flowsheets based on microtechnologycan be further exploited by integrating two unit operations. Returningto the metaphor of the stack of icons representing a unit operation,this aspect would essentially ‘shuffle’ the decks of two or more unitoperations. The same approach for ‘wiring’ this combined stack withcombinations of co-current, counter-current, and crosscurrent flowstreams applies again. For example, two consecutive reactions could becarried out with one reactant stream flowing through a series ofinterleaved, alternating reactor cells, while some of a second reactantcould be added to each reactor pair in parallel, thereby giving rise toa crosscurrent configuration. Given the ability to achievenon-equilibrium product mixtures (e.g., reductions in the formation ofsecondary reaction products) with extremely fast residence times inmicrochannels, a three-dimensional process design approach will open upnew opportunities for managing selectivity while achieving highconversion. The same three-dimensional integration approach is possiblefor other combinations of reactors, separations, and heat exchange.

One subclass of ortho-cascaded microsystem process networks is themanagement of thermal energy. In this subclass, the main advantage isthe distribution of energy addition or energy removal. Some possiblebenefits include reducing exergy losses by reducing required temperaturedriving forces, enabling use of lower temperature materials ofconstruction by lowering temperatures, and increased energy efficiencyby facilitating better heat integration.

One example of a practical application of the subject invention isillustrated in FIG. 1 for steam reforming of hydrocarbons to producesynthesis gas, consisting predominantly of hydrogen, carbon monoxide,and carbon dioxide. This is a highly endothermic reaction, which isconventionally operated at temperatures ranging from less than about500° C. for methanol to temperatures over 1000° C. for methane.Microchannel reactors having reactor channels interleaved with heatexchange channels for delivering heat directly to where the reaction isoccurring, achieves more isothermal operation. The heat can be deliveredusing a gas stream typically heated by combustion to a temperature abovethe reformer operating temperature prior to entering the reactor. Theheat duty required by the reactor is satisfied by the combination of hotgas mass flow rate, combustion operating temperature, and the heatexchange effectiveness in the reactor. This can lead to a trade-offbetween having a higher combustion temperature versus a higher massflow. High temperatures can require that the combustor and reactor beconstructed from expensive high-temperature alloys and lead to largertemperature gradients within the reactor, while higher mass flow causesto energy inefficiencies and larger devices and/or higher pressuredrops. A 3-dimensional flowsheet is constructed to mediate thistrade-off between high temperature and high mass flow. The same hot gasstream is used to heat multiple reactor cells that are operating inparallel by injecting and combusting additional fuel before each cell inorder to increase the temperature before entering the cell. The endresult is that the maximum temperature of the combustion gas stream islowered without having to increase mass flow. Not only does this allowlower temperature alloys to be used, but also improves overall systemefficiency as depicted in FIG. 2. Another potential benefit is betterheat transfer by reducing the gas velocity, thereby allowing for smallerdevices with lower pressure drop.

A second subclass of ortho-cascaded microsystem process networks is themanagement of mass flows. Here, the addition or removal of mass isdistributed within the network. Advantages from using ortho-cascading ofmass flows can include adjustment of reactant ratios, enhanced masstransfer within a unit operation, or avoidance of thermodynamicpinch-points.

An example of practical utilization of this subclass is adjustment ofthe ratio of reactants entering a reactor. Potential side reactions insteam reforming of hydrocarbons are coke formation from cracking or fromcarbon monoxide. One approach for controlling these side reactions is toadd steam in excess of the stoichiometric requirement; steam to carbonratios in the feed are typically 3:1 or higher. The modularity ofmicrochannels facilitates the ability to operate at a higher effectivesteam to carbon ratio by distributing the injection of hydrocarbonreactant. For example, starting with a steam to carbon ratio of 6:1, theratio will rise with conversion to 11:1 at 50% conversion of hydrocarbonto carbon monoxide (assuming no water shift to CO₂ or methaneformation). Adding 42% more hydrocarbon at this point, lowers the steamto carbon ratio back to 6:1. Repeating this process two more times afterachieving 50% conversion (adding 38% and 35% more hydrocarbon for thenext two repeats to reduce the ratio back to 6:1) and ending with 90%conversion of the hydrocarbon in the last step, the overall ratio ofsteam added to hydrocarbon added is 3.34:1. However, the lowest steam tocarbon ratio actually encountered by the process is 6:1, and the overallhydrocarbon conversion is 96%. Intermediate hydrocarbon injection couldbe achieved between cells operated in series or within a single cellhaving multiple passes. In the latter case, injection could occur in theheader region between passes. Several key attributes of microchannelreactor technology make this concept practicable. First, the modularityof microchannel reactors enables injection of reactants into thereaction mixture at multiple points without requiring additionalhardware units. Second, the short residence times achieved inmicrochannel reactors implies that distributing reactant addition alongthe flow path does not severely penalize overall reaction kinetics andso the penalty in increasing the total hardware volume is minimized.

Ortho-cascading can also be used to advantage in microsystem processnetworks in the management of the integration of multiple unitoperations. In addition to integrating heat and mass transfer, otherexamples of unit operations that can be cascaded include pumping,compressing, and mixing. Advantages from using ortho-cascading of unitprocess operations can include equilibrium shifting, optimization ofoperating conditions, adjustment of operating conditions, to name just afew.

The following example illustrates the concept of integrating two unitoperations. Again using steam reforming of hydrocarbons as arepresentative process, the reforming reactor is integrated with aseparation device, such as a hydrogen separation membrane as shown inFIG. 3. The overall concept is to improve conversion and selectivity byremoving one of the products between reactor cells that are operating inseries. The example in FIG. 3 shows selective removal of hydrogen usingmembranes, such as the high temperature palladium membranes, betweenreactor cells operating in sequence. Removing the hydrogen causesequilibrium to shift towards higher hydrocarbon conversion, induceshigher water gas shift toward CO₂, and suppresses methane formation.FIG. 3 illustrates the high level of versatility in modular microchannelarchitecture. Because the volumetric flow is decreasing as hydrogen isremoved, the number of parallel cells can be reduced after eachseparation step. Furthermore, high conversion is not needed by any givenreactor in order to achieve high overall conversion. This illustratesthe ability to combine series and parallel processing in order tooptimize overall size and performance.

Ideally, methanol steam reforming is performed at lower temperatures,typically 250° C. to 300° C., while gasoline reforming is done at 650°C. to 800° C. and methane at even higher temperatures. By incorporatingcompact, high-efficiency microchannel recuperators into the conceptdepicted in FIG. 3, the reactors and membranes can be operated at verydifferent temperatures without incurring a severe energy efficiencypenalty. Alternatively, employing low temperature polymer hydrogenmembranes would reduce the maximum membrane operating temperature toabout 150° C., which could only be efficiently accomplished usingrecuperators. By incorporating heat exchangers, the 3D flowsheeting isemployed to integrate three unit operations, reactors, separations, andheat exchange.

All of the concepts described above for steam reforming hydrocarbonscould be incorporated in a wide variety of combinations and processes.FIG. 3 illustrates the option of integrating membrane separations anddistributing the addition of hydrocarbon to adjust steam to carbonratio. Thus, while three sample embodiments of ortho-cascading areillustrated for steam reforming, many other microchemical and thermalsystems are part of the present invention.

Various preferred embodiments involving: size of components, temperatureranges, flow rates, pressures, 3-dimensionality of unit operations, andspecific combinations of unit operations can be seen in the Figures,examples and attached claims.

Exergetic Efficiency

In thermodynamic terms, “work” can be produced when two systems (or asystem and its surroundings), that are not in equilibrium with oneanother, are allowed to come into complete or partial equilibrium withone another. The “exergy” content of a stream, or of a system, is aquantitative measure of the maximum amount of work that could beextracted from this process, and accordingly represents the amount ofwork that could be produced if the process is thermodynamically“reversible”.

The terms exergy, exergy destruction, and exergetic efficiency areperformance-related parameters that help to describe the efficiency withwhich energy transformation is accomplished within components of thesystem.

Exergy destruction is the amount of potential work (exergy) that is lostas irreversibilities occur during an exchange or energy. Exergy can bedestroyed through a number of mechanisms, including chemical reactions(e.g., combustion), through heat transfer across a temperaturedifference, through mixing, through expansion and through friction(e.g., fluid pressure drop).

Exergetic efficiency is calculated as the fraction or percentage of theexergy supplied to a system (or a component) that is recovered in theproduct of the system (or component). Exergetic efficiency is alsoreferred to in some textbooks as the Second Law efficiency, since it ismore closely tied to the Second Law of Thermodynamics than to the FirstLaw. More specifically, the exergetic efficiency of a component,subsystem, or system is the ratio of the change in the exergy content ofthe product stream(s) to the change in the exergy content of theexergy-driving streams (i.e., the streams which deliver exergy to theprocess). For terrestrial applications, the surrounding environment istaken to be standard temperature (273 K) and pressure (1 bar). Whennon-steady state conditions are encountered, the change in exergy of thehardware system must also be included in the calculation. For purposesof the invention, exergetic efficiency is calculated as illustrated inthe examples.

Since exergy is not conserved in any real systems, the exergeticefficiency of a component, subsystem or system can never equal 100%.There are cases where the exergetic efficiency can be less than 0%,however, such as when the exergy of a product stream is less than theexergy of the feed streams. This case is document in Szargut (1984) foran ammonia production plant.

There can be a substantial advantage for an engineer to using the SecondLaw for efficiency considerations. First, efficiency calculations basedon the Second Law are often a better measure of the value (or quality)of the energy potential of a fuel or an stream's enthalpy. As noted byGaggioli and Petit (1997), “the real commodity of value, which thelayman calls energy, isn't the same thing as the energy of science.Rather, the energy commodity is called available energy, or potentialenergy, useful energy.”

The First Law of Thermodynamics is essentially a conservation law.Expressed in mathematical form, the First Law states that in any energytransformation process, energy is always conserved. In contrast, theSecond Law of Thermodynamics is essentially an expression of the effectof irreversibilities on any transformation process, with perhaps themost commonly used parameter being entropy. The Second Law is alsorecognized as an indicator that any energy flow can also be expressedqualitatively in terms of its ability to effect change (e.g., performwork), and that this quality can be degraded or destroyed by the stepsin practical processes. Exergy has been proposed by thermodynamicists asthe quantification of that quality; that is, the exergy content of anystream is a measure of the ability of the energy in that stream toperform work or create change.

Exergy analyses recognize that there is potential work available fromexothermic chemical reactions, likewise that it is exergy that drivesendothermic chemical reactions, and individual reactants and productsare assigned “chemical exergy” values. Typically, a flowstream will havea chemical exergy quantity, that is a function of the chemicals in thestream and its flowrate, and a physical exergy quantiy, that are afunction of the temperature, pressure, and flowrate of the stream. Thesevalues are also a function of the temperature, pressure andconstitutents of the “surrounding environment” of the system. For areactor, exergetic efficiency calculations include the efficiency withwhich exergy is transformed from one form to another.

More generally, the exergy (E) of a stream is typically described asbeing composed of four components (absent nuclear, magnetic, electricaland interfacial effects), separately described as physical exergy(E_(PH)), kinetic exergy (E_(KE)), potential exergy (E_(PE)) andchemical exergy (E_(CH)).E=E _(PH) +E _(KE) +E _(PE) +E _(CH)The kinetic and potential exergy terms are equivalent to kinetic andpotential energy, and can typically be assumed to be small compared tothe other terms. However, this assumption needs to be carefullyconsidered in each case, as there are notable exceptions.

The physical exergy term is typically calculated based upon methodsdescribed in Szargut et. al. Exergy Analysis of Thermal, Chemical, andMetallurgical Processes, Hemisphere Publ. Co. (1988), Moran (1982), andBejan et. al., Thermal Design Optimization, Wiley-IntersciencePublication (1996), and is mathematically based upon the followingexpression for fluid flow within an open system:E _(PH) =H−H _(o) −T _(o)(S−S _(o))where H and S represent the enthalpy and entropy of the fluid stream atthe point of interest, H_(o) and S_(o) represent the enthalpy andentropy of the same stream if brought to the same temperature andpressure as the environment (T_(o) and P_(o)). Derivations of thisexpression are found in a number of texts, including those alreadymentioned. Note that this equation is similar to, but should not beconfused with, the Gibbs function for free energy.

The value for the chemical exergy of a stream is typically assigned froma table of chemical exergies, based upon consideration of thesurrounding environment. Substantial work has been done to definereference chemical exergies for a number of chemicals in terrestrialsettings, and the reader is again referred to the previously citedliterature for additional information.

It is perhaps instructive to examine the method of exergy analysis for asimple case, where a hot gas is used for the purpose of heating a coolgas. In this example, we will assume that a recuperative heat exchangeris operating with two fluid streams, each consisting of the same idealgas with the same mass flow rates, and with constant specific heats andno chemical reactions. Likewise, in this example, we will assume thatthere are no significant changes in potential energy or kinetic energyof either gas stream.

In this case, the previous equation for physical exergy can be restatedas follows:E _(PH) /mc _(p) T _(o)=[(T/T _(o))−1−ln(T/T _(o))]−[ln(P/P_(o))^(k/(k−1))]where m is the mass flow rate or molar flow rate for the gas, c_(p) isthe specific heat of the gas, and k is the ratio of specific heats forthe gas. The first bracketed term is the contribution of the temperatureof the gas to the physical exergy of the stream, and the secondbracketed term is the contribution of the pressure of the gas to thephysical exergy of the stream. In each bracket, the exergy contributionis specific to the state of the environment, which is consistent withthe definition of exergy as being the amount of work that couldtheoretically be extracted from the stream if it were to be reversiblybrought into equilibrium with the environment.Assuming that the inlet and outlet conditions of the stream to be heatedare T₁, P₁ and T₂, P₂, respectively, and assuming that the inlet andoutlet conditions of the heating stream are T₃, P₃ and T₄, P₄,respectively, then since there are no changes in kinetic energy,potential energy, or chemical exergy, the expression for the amount ofexergy that is given up by the hot stream is:E ₃ −E ₄ =mc _(p) T _(o){[(T ₃ /T ₄)−1−ln(T ₃ /T ₄)]−[ln(P ₃ /P₄)^(k/(k−1))]}where k is the ratio of specific heats. Likewise, the exergy increase ofthe cold stream is:E ₂ −E ₁ =mc _(p) T _(o){[(T ₂ /T ₁)−1−ln(T ₂ /T ₁)]−[ln(P ₂ /P₁)^(k/(k−1))]}For a reversible process, there would be pressure drop within eitherstream, and there would be no temperature difference between the streamsat any point at which heat is exchanged. That is, we would have T₁=T₄and T₂=T₃. However, reversible processes exist only in textbooks, and infact there can be no heat transfer without temperature differences.Therefore, for a realistic recuperative heat exchanger, some exergy isdestroyed (E_(DES)) and this can quantitatively can be expressed as:E _(DES)=(E ₃ −E ₄)−(E ₂ E ₁)In accordance with the Second Law of Thermodynamics, some exergy isdestroyed in the recuperative heat exchanger example, just as somequantity of exergy must be destroyed in any energy transformation. Inthe example, the loss in exergy is physically accomplished throughfriction (pressure drop) and heat transfer (against a temperaturedifference). The latter feature is further demonstrated by theobservation that T₂<T₃ for real heat exchangers.

When no chemical reactions or other exergy transfers are accomplished bya component, the exergetic efficiency of a device is typicallycalculated to be the increase in physical exergy within the productstream divided by the decrease in physical exergy within the otherstream. For this case of the recuperative heat exchanger, the exergeticefficiency (or Second Law efficiency) is therefore:ε_(second law)=(E ₂ −E ₁)/(E ₃ −E ₄)This example further demonstrated another previously mentioned feature:All real-world processes result in some degree of degradation in theuseful work that could be accomplished by any energy stream.

There is perhaps no better example than the case of a simple, steampowerplant, producing electricity from a fossil fuel. In this case, thechemical exergy of the fuel is converted to heat in the furnace at ahigh temperature, perhaps exceeding 2000° C. with this heat being usedto produce superheated steam, typically at less than 600° C. If weassume that the environment is at about 25° C., then applying theefficiency derivation of Carnot for heat engines, we would expect that areversible heat engine could extract useful work at a First Lawefficiency ofε_(first law)=[1−(25+273)/(600+273)]×100%=65.8%However, conventional steam powerplants typically produce electricity atFirst Law efficiencies of only about 35-40%. This supports twoobservations: 1) that the conversion of the fossil fuel's chemicalexergy to physical exergy within the combustion gases and then tophysical exergy within the superheated steam, at only 600° C., resultsin the destruction of 34.2% of the chemical exergy of that fuel, and 2)that there is significant additional destruction of exergy in theremainder of the steam powerplant. This fact is of course well known toengineers, who also appreciate that the largest source of exergydestruction in the steam powerplant is in fact the irreversible transferof heat from the combustion gases to the steam, which is typically onlyheated to several hundred degrees.

In general, combustion of a fuel is a source of large irreversibilities,and therefore it is typically accompanied by the destruction of asignificant quantity of the chemical exergy of the fuel. However, theseirreversibilities can be reduced through preheating of and minimizingthe use of excess air. In this way, heat transfer across temperaturedifferences, a significant source of exergy destruction, can beminimized.

On the value of the exergetic efficiency metric, Bejan et. al. statethat:

-   -   The exergetic efficiency . . . is generally more meaningful,        objective, and useful than any other efficiency based on the        first or second law of thermodynamics, including the thermal        efficiency of a power plant, the isentropic efficiency of a        compressor or turbine, and the effectiveness of a heat        exchanger. The thermal efficiency . . . is misleading because it        treats both work and heat transfer as having equal thermodynamic        value. The isentropic turbine efficiency, which compares the        actual process with an isentropic process, does not consider        that the working fluid at the outlet of the turbine has a higher        temperature (and consequently a higher exergy that may be used        in the next component) in the actual process than in the        isentropic process. The heat exchanger effectiveness fails, for        example, to identify the exergy waste associated with the        pressure drops of the heat exchanger working fluids.

In steam powerplants, First Law and Second Law efficiencies are oftenabout the same, in part because the chemical exergy of a fuel is aboutequal to the Heat of Combustion for the fuel. However, for manycomponents and systems, the First Law and Second Law efficiencies areoften very different, as can be observed in Table 1 (from Kenney, EnergyConservation in the Process Industries, Academic Press 1984). TABLE 1Comparison of First and Second Law Process Efficiencies (%) UnitOperation (or process) First Law Second Law Residential heat (fuel) 60 9Domestic water heater (fuel) 40 2-3 High-pressure steam boiler 90 50Tobacco dryer (fuel) 40 4 Coal gasification, high Btu 55 46 Petroleumrefining ˜90 10 Steam-heated reboiler ˜100 40 Blast furnace 76 46Table 1 also reflects the fact that a First Law efficiency calculationis 100% if there is no loss of energy to the environment. The Second Lawefficiency calculation, however, includes the realization thatirreversibilities in the process reduce the ability of the energy flowto effect change (support work or chemical conversions/separations).

Although exergy is thermodynamically equivalent to the maximum availablepotential work that could be obtained, there is significant value tousing this metric as a measure of value and efficiency for the processindustries. For one thing, process plants are often extremely energyintensive, containing significant heat exchanger networks, and a systemthat produces a high value chemical product (whether it be a fuel ornot) is often more valuable if it does so with reduced energyrequirements compared to an alternate system that requires theconsumption of larger quantities of energy. Increased energy efficiencylikewise corresponds to a reduction in the amount of fuel (or, commonly,feedstock petrochemicals) required in a process plant, with similarreductions in atmospheric emissions, including greenhouse gases.

The inventors have therefore attempted to design a compact microchanneldevice to include exergetically efficient recuperative heat exchangers(to preheat reactants), and exergetically efficient reactors andcombustors. For the realization of a improved energy efficiency, theinvention utilizes recuperative microchannel heat exchangers thatoperate with high exergetic efficiency, which includes consideration ofthermodynamic irreversibilities associated with both heat transfer andpressure drop. Here, the chemical exergy of the flowstream isunimportant, as there is no transfer of chemical exergy content (due tothere being no chemical conversion taking place). Hence, the destructionof exergy in a well insulated, recuperative microchannel heat exchangercomes primarily through a) heat transfer against a temperaturedifference (resulting in a degradation of the quality of heat availablein the stream, or alternately, a reduction in the amount of work thatcould be performed if the stream provided energy for a heat engine) andb) pressure drop due to fluid friction (resulting in a reduction in thepossible work that could be performed by the stream).

Conventionally, heat exchangers in microsystems have been designed toachieve First Law efficiency. In the present invention, by analyzingfactors important for high exergetic efficiency such as short heattransport distances, large temperature differences and low pressuredrops, we have designed a microsystem having an exergetically efficientheat exchanger. Preferably, the heat exchanger is at least 80%, morepreferably at least 85%, exergetically efficient. In some embodiments,the heat exchanger is between about 80 and about 95% exergeticallyefficient. In the present invention, exergetic efficiency is measured bythe example below. Examples of some structures, conditions andmodifications that can be applied in exergetically efficientmicrosystems are discussed in the following sections.

System Construction

a) System overviews. Flow charts of two systems utilizing steamreforming are illustrated in FIGS. 4 and 5. In FIG. 4, water ispreheated in preheater 10 prior to being vaporized in water vaporizer20. Liquid fuel, such as iso-octane, is vaporized in fuel vaporizer 30.Recuperator 40 heats the fuel and steam. The fuel-steam mixture flowsinto steam reformer 50. Since steam reforming is a highly endothermicreaction, hot gases from combustor 60 are passed through the steamreformer to heat the reactants in reformer 50. After transferring heatin the steam reformer, residual heat in the combustants is captured bypassing the combustants through recuperator 70 where air is heated, andwater vaporizer 20. Product gas 55 from the steam reformer passesthrough recuperator 40 where heat is transferred to the fuel mixture.Additional heat is transferred in fuel vaporizer 30 and water preheater10. The foregoing components comprise a complete microchannel syngasproduction unit. If the system is used with PEM fuel cells, for powergeneration, then the synthesis gas (also called syngas) product willrequire additional processing prior to introduction to the internals ofthe PEM fuel cell 82, as CO is a catalyst poison for most PEM fuelcells, and as the overall system efficiency can be increased if the COis converted to CO₂ within a water-gas-shift reactor, which alsotransfers the energy content of the CO into additional H₂ content, whichis the fuel for a PEM fuel cell. Application with PEM fuel cells isdepicted within FIG. 4.

The objective of the compact microchannel steam reforming unit,schematically illustrated in the left side of FIG. 4, is the productionof hydrogen-rich gas from hydrocarbon feedstock, for applications wherethere are advantages through the realization of compact, lightweighthardware for hydrogen gas production. Examples include power productionfor both stationary and mobile applications (e.g., vehicularapplications), where the product gas is subsequently processed asnecessary in accordance with the fuel requirements of a fuel cell andfor the production of chemicals where hydrogen or synthesis gas is afeedstock to the chemical process that is employed. This flowsheetindicates steam reforming of liquid hydrocarbons, however, otherembodiments could utilize gaseous hydrocarbons (e.g., methane).

A compact microchannel steam reforming unit can include:

-   -   Microchannel catalytic steam reforming reactors (1 or        more)—which contain integral microchannel heat exchangers so        that the endothermic steam reforming reaction receives its heat        from the combustion gas stream    -   Microchannel recuperative heat exchangers (2 or more)—which        provide for efficient preheating of the stream reforming stream        and the combustion stream, utilizing (respectively) the stream        reforming products and the combustion products as the source of        heat from each.    -   Microchannel vaporizers for generating steam and gaseous        hydrocarbons. In the system configuration that is depicted in        FIG. 4, the heat of vaporization for both streams is essentially        provided by the latent heat of the combustion (product) stream,        however other system configurations are possible where some of        the heat of vaporization of one or both streams are provided, in        part, from the latent heat that is present in the steam        reforming (product) stream, or where the heat of vaporization        for either or both the water and the hydrocarbon stream are        separately provided.    -   Microchannel or other compact units for the provision of heat to        the heat-providing stream (e.g., a compact combustion unit).

A highly efficient recuperative microchannel heat exchanger in thissystem is identified as Recuperator 1, which has been designed tofunction with an exergetic efficiency exceeding 85%. The exergy analysisof this component and the other components of the system is presented inthe following section.

If the unit is used with solid oxide fuel cells (SOFCs), then noadditional processing of the syngas product is required. In this case, afurther advantage is realized, wherein the heat from the hightemperature SOFC is at a high enough temperature so that it can be usedas process heat for the endothermic steam reforming reactor. FIG. 5shows one possible system configuration of the Compact MicrochannelSteam Reforming Unit when used with a solid oxide fuel cell. In thissystem, fuel is vaporized in vaporizer 210, combined with steam fromwater vaporizer 220 and heated in recuperator 230. The heated stream isreacted in reformer 240. The reformer 240 is integrated with a combustorthat supplies additional heat. The resulting reformate is heated to 900°C. to 1100° C., preferably about 1000° C., in recuperator 250 and passedto solid oxide fuel cell 260 to generate electrical power 270. Wasteheat from the solid oxide fuel cell can be recovered by passing the hotwaste gases back through the system as shown in FIG. 5.

In general, the design capacity of the compact microchannel synthesisgas production unit could be increased by either increasing the size andthroughput of each component, proportionally, or by utilizing a designfor the system where multiple process trains are processed in parallel,or by some combination of both. For the case where multiple processtrains are processed in parallel, there are additionally other featuresthat can support the energy efficient production of synthesis gas, suchas by operating each train (and component) at a point that is near or atits most efficient conditions, and digitally turning up (or down) theproduction rate by turning on (or off) individual process trains.

It is also apparent that individual components can be separately plumbedinto the system without exceeding the contemplated bounds of theinvention, so that not all components have to be part of the sameintegral hardware unit.

Hydrocarbon feedstocks for which this unit is considered includeintermediate-length-chain hydrocarbons (e.g., iso-octane) andshort-chain hydrocarbons (e.g., methane, ethane, etc), plus alcohols(e.g., methanol, ethanol, etc). The production of syngas from complexmixtures (e.g., gasoline or diesel fuel) is contemplated; however,additional unit operations may be required in order to deal withconstituents that are problematic for reforming. Example constituents ofconcern include sulfur-containing compounds, aromatics, and detergents.

For the heat-providing stream, energy can be provided through either a)the combustion of a portion of the hydrocarbon feedstock, a portion ofthe steam reformer gas product, a fuel-containing stream that comes fromdownstream processing (e.g., the anode effluent of a fuel cell), or someother fuel stream or b) process heat from some other system (such as ahigh temperature, solid-oxide fuel cell), or c) some combination of a)and b) together.

Many other embodiments are contemplated within the scope of thisinvention. For example, highly efficient recuperative microchannel heatexchangers could be utilized as part of a CO₂ collection system (whichis based on temperature swing absorption) and for preheating ofreactants to and cooling of products from the reactors.

b) Shim designs and exergetic efficiency calculations. The followingsection describes, with accompanying figures, shim designs that werefabricated and tested. The section also includes some generalizeddiscussion of component design, modifications, and operating parametersthat are applicable in a wide variety of devices and systems. Althoughcomponents are described as part of a steam reforming apparatus, itshould be understood that the following designs and other descriptionsare not limited to the steam reforming process, but are generallyapplicable to systems employing microchannel components.

Example: Highly Exergetic Recuperative Heat Exchanger

A microchannel recuperative heat exchanger was designed, fabricated andexperimentally demonstrated to be highly efficient, with an exergeticefficiency (considering both heat transfer against a temperaturedifference and pressure drop) that was demonstrated to be greater than80%. The microchannel heat exchanger was designed to achieve a desiredheat transfer effectiveness of 0.85. The design is described below.Several conservative assumptions were made in the initial design modelincluding:

-   -   calculating the Nusselt number for both constant temperature and        constant heat flux boundary conditions and taking the minimum        (constant heat flux is the more correct boundary for the        recuperator)    -   giving full weight to longitudinal conduction in the metal at        the outside edges of the exchanger    -   using an abbreviated pressure drop calculation with an extended        entrance section to estimate shim pressure drop rather than a        short entrance followed by an expanding region.

A microchannel recuperative heat exchanger was constructed thatconsisted of 10 pairs of shims consisting of 20 mil (0.50 mm) shimspartially etched to a depth of 10 mil (0.25 mm). The entire device wasconstructed of 316 stainless steel. The shim design for shim “A” isshown in FIG. 6. The “B” shim is the mirror image of the “A” shim exceptthat it connects to the alternate set of header holes. The device wascovered top and bottom by 50-mil (1.25 mm) thick cover plates. The shimswas assembled with the A and B plates facing each other: Top Plate, A,B, A, B . . . A, B, Bottom Plate. The top plate has 2 header holes thatalign with the header holes 610, 620 in Plate A and the bottom plate has2 header holes that connect to the header holes in Plate B. The twogases flow countercurrent in the heat exchange section and in theheaders flowing in the stack direction flow in the same direction (e.g.the initially hot gas enters through headers 610 and leaves throughheaders 620 while the initially cold gas enters through headers 630 andleaves through headers 640). The header holes were etched from bothsides with all other areas on the back side masked off. Prior tobonding, a nickel coating was applied over the steel plates and the unitwas diffusion bonded. The tolerance on the alignment pin holes could notbe achieved in the etching process so the holes were made slightlyundersized and manually reamed to the correct size. External headersconsisting of one half of a ⅜″ tube (0.0325 inch wall) with a ⅜″ tube“tee” leg were welded onto the two stacks. The external headers combinethe two internal header holes and provided for a connection to the teststand. A full tubing tee leg was centered on the shim width.

The design included a 100 mil (0.25 cm) bonding perimeter in all regionsthat require a seal. In the shim entrance regions, which are supportingbonding in the shim directly above, two 20-mil wide ribs were placed inthe 250 mil opening dividing it into three 70 mil passages. Thisapproach successfully sealed the device.

The internal support ribs in the heat exchange section were spaced atapproximately quarter-inch intervals. Leak-proof bonding is not requiredin the region of the internal ribs so the major function of the ribs isto limit the deflection of the wall due to differential pressures. Thefour slanted ribs are intended to help distribute flow. In the sectionwith flow directed along straight ribs, the support ribs areintermittent. This allows flow redistribution if needed. It is believedthat if flow is not sufficiently uniform, heat transfer effectivenessmay suffer significantly. The interrupted support rib is expected tohave a small positive effect on heat transfer and introduce a smallpenalty in flow friction.

The shim stock from slice of stainless indicates that the 0.020 inchthickness material has a tolerance of ±0.003 inch. A single sampling ofone shim A and one shim B (6 measurement points on each) indicated thatthe shim material was about 20.6 mil on the shim A sample and 20.9 milon the shim B sample, which is well within the 3 mil tolerance. Thedepth of the etching on these two shim samples averaged 10.0 mil on shimA and 9.8 mil on shim B. No measurements were made of a standardthickness gauge for comparison and the calibration of the micrometer wasexpired so this data should be considered as indication only. Theoverall thickness of the recuperator after testing was 0.4973 inchcompared to a target thickness of 0.5000 inches (2*0.050″+20*0.020″).

The size of the bonded unit was measured to be 3.020 inches by 1.50inches by 0.50 inches (2.265 in³, or 37.12 cm³). Pressure taps andthermocouples were located at the tees into which each exchangerinlets/outlets are connected. Kao-wool insulation was applied to theexchanger and wrapped in aluminum tape.

Heat exchanger tests were conducted by selecting the desired nitrogenflow and furnace temperature and then waiting for temperatures on theexchanger to stabilize. All data was taken at steady state. The timerequired to achieve a steady state condition was between 15 and 45minutes depending on temperature and flow rate. The slow dynamics arebelieved to be primarily due to the long flowpath and thermal massassociated with the heated gas flow path prior to entering theexchanger. This could be modified to allow more rapid collection ofdata. No attempt was made to sort out the dynamic response of therecuperator itself from the data. A total of 21 steady-state testconditions were recorded and evaluated with full insulation on theexchanger, with heating rates being varied from 86 watts to 943 watts.The highest thermal power density (heat rate per unit hardware volume)was therefore 25.4 watts/cm³. The system was operated with nitrogen asthe working fluid at flow rates varying from 30 to 126 slpm at inletpressures for the cold fluid ranging from 4.76 to 30.95 psig. The samestream was used for both the cold fluid and the hot fluid, with the coldfluid being additionally heated after leaving the heat exchanger beforebeing returned to the unit and serving as the hot fluid. Inlettemperatures for the cold fluid were generally about 23-26° C. Outlettemperatures for the cold fluid were raised to various levels, rangingfrom 188° C. to 473° C. The inlet temperature for the hot fluid wasvaried from 220-575° C. with the outlet temperature varying from 61-145°C.

Upon first examination of the data it was clear that the model wasslightly conservative in predicting effectiveness (i.e. the actualexchanger effectiveness was higher than the model prediction). The firstadjustment made to the model prior to reducing the data was to force themodel to use the constant heat flux boundary condition in estimating theNusselt number rather than the constant temperature boundary condition.The constant heat flux boundary is clearly the more correct boundary forthe recuperator and the minimum Nusselt number was used simply as aconservatism in design.

The model predictions of effectiveness (using the constant heat fluxboundary condition) are plotted against the observed effectiveness inFIG. 7. The area for heat transfer in the model is taken as 2.80×10⁻²m², consisting of a region 4.755×10⁻² m long and 3.099×10⁻² m wide. Thisassumed area neglects the area in the entrance regions to the headers.

The exergetic efficiency of the microchannel heat exchanger wascalculated from the data by using the technique for ideal gases (Bejan,Tsatsaronis, and Moran, 1996), taking into account both temperature dataand pressure data, and are shown in FIG. 8. FIG. 9 additionally showsthe amount of exergy destruction as a function of heating rate

A preferred embodiment of a steam reformer system is illustrated inFIGS. 10-15. This system is called “the Compact Microchannnel SteamReforming Unit” in the following discussion. A schematic overall view ofthe apparatus 101 is illustrated in color in FIG. 10 (a black and whiterendition is included as FIG. 18). Fresh air enters through airpreheater inlet 106 and is warmed in the air preheater (gray block). Theair is split into four streams moving through conduits 110 (green) tofour recuperators 124, 140 (pink). Each of these recuperators contains arecuperative heat exchanger 120 and water vaporizer 122. Hot air exitsrecuperator 120 into header 112 (gray) and is mixed with fuel in tube102 (red) which travels to combustor 104 (red). The resultingcombustants travel through header 118 to reactor 114 (blue). The gasruns in series through four cells. In each cell, heat from thecombustants is transferred to drive the endothermic production ofhydrogen. At the reactor, the combustant stream is connected in serieswhile the reactant stream is connected in parallel. After passingthrough the first cell, the gas leaves the reactor 114 through header116 (purple) and hydrogen gas is injected through an inlet (not shown)in the header 116. The hydrogen gas spontaneously ignites, adding heatto return the gas to the temperature at which the gas first entered thereactor. The gas then reenters the reactor to again drive the formationof hydrogen. After passing through the fourth cell, the combustant gasesexits through a header 134 (pink) where it is split into four separatestreams and used to vaporize water. The combustant streams arerecombined in header 108 and used to warm up air in the air preheaterbefore being exhausted.

In the other fluid stream, water (which optionally could come through apreheater) comes into each of four water vaporizers 122, is converted tosteam, and passes through headers 126 (blue) to fuel vaporizer 132(yellow) where the steam is mixed with fuel from a fuel inlet (notshown) and each of the four mixtures passes through a header 138(yellow) into a cell of reactor 114 where hydrogen is produced. In apreferred embodiment, both the reformate stream and the combustant (heatexchange) fluid streams exit the reactor at about 750 C. Heat from thereformats stream is recovered in recuperator 130 and vaporizer 132 andthe reformate exits the device (through gray tubes).

The design for the air preheater was an interleaved microchannel heatexchanger 1000 that consisted of 10 pairs of shims consisting of 20 mil(0.50 mm) shims partially etched to a depth of 10 mil (0.25 mm). Theentire device was constructed of 316L stainless steel. The shim designfor shim “A” is shown in FIG. 12 a. The “B” shim, shown in FIG. 12 b, isthe same as the “A” shim except that it connects to the alternate set ofheader holes. The device is covered top and bottom by 50-mil (1.25 mm)thick cover plates. The shims are assembled: Top Plate, A, B, A, B . . .A, B, Bottom Plate. The top plate has 20 header holes that align withthe header holes 1010, 1020 in Plate A and the bottom plate has 20header holes that connect to the header holes in Plate B. The two gasesflow countercurrent in the heat exchange section and in the headers(e.g. the initially hot gas enters through headers 1010 and leavesthrough headers 1020 while the initially cold gas enters through headers1030 and leaves through headers 1040). The shims were 7.1 inch long and3.0 inch wide. The heat exchanger was bonded as described above.

A similar design for another interleaved heat exchanger is illustratedin FIGS. 13 a and 13 b. In this exchanger, there were 20 pairs of shimsconsisting of 31 mil (0.78 mm) shims partially etched to a depth of 10mil (0.25 mm), that is, a channel depth of 10 ml. These shims measured2.9 inch wide by 3.3 inch long (excluding alignment tabs). This heatexchanger is not illustrated in FIG. 10, but could, for example, beattached to the reformate carrying tubes to condense water.

The shim design for a combined recuperative heat exchanger 130 and fuelvaporizer 132 is illustrated in FIGS. 11 a and 11 b. These shims were 17mil thick with an etch depth of 6 or 7 mil. Each shim is 1.5 inch wide(excluding assembly alignment holes 152, which are cut off afterbonding) and 5.34 inches long. The shims shown in FIGS. 11 a and 11 bwere stacked in alternating layers capped by end plates having inlet andoutlet fluid headers. The flow microchannels were structurally supportedby 10 mil thick lands. In the heat exchanger layer, hot reformate gasenters through inlets 162, and travels through the layer to outlet 150.While fluid flow flows within a single microchannel, that fluidparticipates in three unit operations. In the illustrated embodiment, inregion 158 heat is transferred to reactants prior to entering thereactor, in region 156 heat is transferred to fuel vaporizer 176, and inregion 154, 172 heat is transferred to water. The heated water exitsthrough outlet 170. Vaporized fuel exits through outlets 174. Reactantsenter through inlets 178 and exits through outlets 160. Preferably thereare at least two layers with a fuel vaporizer and at least one heatexchanger layer, more preferably there are at least three layers with afuel vaporizer and at least two heat exchanger layers. Each of thelayers preferably has a thickness of between 0.1 and 1 mm. While thedescription refers to a steam reforming process, it should be recognizedthat the inventive concepts apply to a wide variety of reactions andunit process operations.

Shims for a combination water vaporizer 120/heat exchanger 122 are shownin FIGS. 14 a-14 d. In this microchannel device 124, the etched face ofthe shim illustrated in FIG. 14 a was placed adjacent to its mirrorimage (the etched face of the shim illustrated in FIG. 14 b) to form ashim pair A. The mirror image shims shown in FIGS. 14 c and 14 d weresimilarly matched to form shim pair B. Shim pairs A and B werealternatively stacked (A-B-A-B- . . . ) with 10 A pairs and 11 B pairs.In the air heat exchanger portion of this device, cool air from the airpreheater enters through inlets 204, move through microchannels 208,where the air is heated, and hot air exits outlet channels 200 afterwhich the heated air flows to the combustor. Water enters header 218through inlet 214. The water passes through laser machined orifices 216and into microchannels 210, where the water is converted to steam andexits through steam outlets 206. Several features are worth noting inthis construction. Lands 220 can be provided for structural support;however, it is preferred that the lands have gaps that help equalizepressure in the microchannels, especially in regions where microchannelsare curved. The orifices help provide even flow through all themicrochannels and reduce the incidence of water spurting through themicrochannels. In a preferred embodiment, flow microchannels for a watervaporizer have a height of about 100 to about 2500 micrometers; a widthof about 1.3 to about 13 millimeters; and a length of about 1 to about30 centimeters. A heat exchange fluid, such as a combustant stream,flows through the shims shown in FIGS. 14 c and d. The hot combustantsenter through inlets 202, passes through microchannels 226 and 224 tocombustion exhaust 212. In region 222, heat is transferred to air, whilein region 220, heat is transferred to a water vaporizer—thusillustrating an example in which a heat exchange fluid can be used formultiple, separate unit operations without leaving the same flowmicrochannel. While two specific unit operations are illustrated, itshould be recognized that this technique can be used efficiently for anydesired combination of unit operations. The top and bottom plates (notshown) were 0.048 inch thick steel plates having header holes for waterinlet, steam outlet, heat exchange fluid inlets and outlets, and airinlets and outlets.

The shim construction for the reactor 114 is shown in FIGS. 15 a-e. Thereactor contained 75 reaction chamber layers alternating with 76 heatexchanger layers. Each reaction chamber layer consisted of a pair ofmirror-image reaction chamber shims 300 separated by a spacer shim 340.The spacer shim had a 12 mil thickness. Each heat exchanger layerconsisted of a pair of mirror-image heat exchanger shims 350. Coverplates 312 were welded on to create reactant channel 302. Reactionchamber 314 was formed by etching 5 mils into the shim while leaving aseries of struts 322 and 324. The reaction chamber 300 and heatexchanger 350 shims were 20 mils thick. Each of the four identicalreaction chambers was about 2 inches by 2 inches. The narrower, 5 milstruts 322 support catalyst strips while the thicker struts 324 alignwith wires 342 to provide structural supports. The wires 342 had athickess of 12 mil and a width of 10 mil. The “X”s in FIG. 15 c indicateempty spaces. These spaces are occupied by strips of catalyst felts (2.1inch long×0.25 inch wide and 10-12 mil thick). Preferred catalystmaterials are described in U.S. Pat. Nos. 6,440,895 and 6,616,909incorporated herein by reference. The heat exchanger shims containedcombustion gas inlets 354 and combustion gas outlets 352, and four setsof flow microchannels 356. The endblocks are usually thicker than theindividual shims, typically 0.25 to 0.35 inch thick. One end block is afeatureless metal sheet. The other end block 360 is illustrated in FIG.15 e.

During operation, reactants enter from the reactant channel 302 intoreaction chamber 314. Products are formed in the reaction chamber andflow out through outlets 304. At the same time, combustant gases enterthe heat exchanger layers through inlets 308, 354, flow through heatexchanger microchannels 356 and exit through outlets 352, 306, 310.While the flow of reactants and products are in parallel, the flow ofcombustant gases is in series. In one preferred embodiment for steamreforming, combustion gases enter holes 362 at 725 C move through theheat exchanger and exit holes 364 at 650 C. The gases are carriedthrough an exterior pipe (not shown) and hydrogen gas is injected intothe pipe to raise the temperature of the gases to 725 C beforereentering the reactor through inlet 366. This process is repeated untilthe combustant gases exit the fourth cell of the reactor at outlet 368at 650 C and travel on to recuperative heat exchanger 120.

The reactor provides numerous desirable characteristics includingcompactness, durability and the ability to conduct a thermal reactionover a relatively narrow temperature range. The reactor provides anintegral design having multiple reaction chambers on a single layer incombination with heat exchange. In preferred embodiments, the heatexchange layers and the reaction chamber layers each have a thickness of0.1 to 2 mm. In another preferred embodiment, more than 3, morepreferably more than 10, alternating layers of heat exchangers andreaction chambers are combined on a single device. It can be seen thatin a preferred embodiment, heat exchanger fluid flows from one heatexchanger to a second heat exchanger within the same integrated device.The reaction chambers preferably have a height, from top to bottom ofthe reaction chamber of 0.05 to 1 mm—which is advantageous for heat andmass transfer. The reactors can also be characterized by theirproperties such as the reaction productivities per unit volume and/orthe thermal power density (in W per cc) achieveable in these systems.

An exergy analysis was performed of the Compact Microchannnel SteamReforming Unit, based on a ChemCad simulation of the system assumingthat it is well-insulated (that is, no heat losses from individualcomponents to the environment). In general, the method of Bejan,Tsataronis and Moran (1996) was used, with ChemCad supplying thefollowing: temperatures, pressures, mass and molar flow rates, byconstituent chemical, for reactants and products (including the heatingstream), enthalpy and entropy differences for each fluid on acomponent-by-component basis, heat of reactions (for combustion andsteam reforming), specific heat values per stream, on a component bycomponent basis, molecular weights, and component heat rates (watts).The physical characteristics of the environment (or “dead state”) wastaken to be 25° C., 1 bar. Chemical exergies were taken from Table C.2.of Bejan, Tsataronis and Moran (1996).

For the reformer only, the entropy difference of the reforming streamwas calculated using ideal gas rules and the assumption of specificheats. The system that was modeled included an air preheater. Formixers, entropy differences and exergetic destruction values werecalculated by the method described in Bejan (1996).

As previously noted, the exergetic efficiency (ε_(2nd Law)), or SecondLaw Efficiency, of a component, subsystem, or system is defined to bethe ratio of the change in exergy of the product stream(s) to the changein exergy of the exergy providing stream(s).

Example Exergy Analysis of the Microchannel Steam Reforming Unit

Calculations were performed for the compact microchannel steam reformingunit, designed to provide a hydrogen-rich stream for a 10 kWe fuel cell,with input assumptions including that the iso-octane and water streamsare fed into the system at 5 bar, and the air stream is fed into thesystem at 2.5 bar. Individual cases included assumptions of substantialpressure drops and negligible pressure drops within individualcomponents. It was further assumed that each component and connectingpipe are substantially insulated, so that heat losses to the environmentare negligible.

As with the current design, the reforming stream is split into fourparallel trains. The combustion stream, which provides heat for theendothermic steam reforming reaction, likewise has four parallel processtrains, but with the mixer/combustor/steam reformer sections beingortho-cascaded. For the exergy calculations, the surrounding environmentwas likewise taken to be standard temperature and pressure (i.e., 298 K,1 bar).

FIGS. 16 and 17 present the Enthalpy Band and Exergy Band diagrams forthe case with negligible pressure drop. Except where otherwise noted,the following description of the results is for the case where pressuredrops are negligible.

FIG. 16 specifically presents the chemical and physical exergies foreach point of each stream. Exergy values for the reforming stream arethe same for each steam reformer, and are indicated only for the streamthat passes through Steam Reformer 4. The exergy destruction estimatesfor each component are likewise identified.

The overall exergetic efficiency of the Compact Microchannel SteamReforming Unit can be calculated through examination of the chemical andphysical exergies of the heating and reforming streams, from FIG. 16.For the heating stream: $\begin{matrix}\underset{\_}{{Input}\quad{Streams}} & \underset{\_}{{Output}\quad{Stream}} \\{E_{{CH}\text{-}{air}} = 1147} & {E_{{CH}\text{-}{combprod}} = 695} \\{E_{{PH}\text{-}{air}} = 417} & {E_{{PH}\text{-}{combprod}}\underset{\_}{= 1417}} \\{E_{{CH}\text{-}{fuel}} = 5407} & {\quad 2112} \\{E_{{CH}\text{-}{fuel}} = 1110} & \quad \\{E_{{CH}\text{-}{fuel}} = 1110} & \quad \\{E_{{CH}\text{-}{fuel}}\underset{\_}{= 1110}} & \quad \\{\quad 10301} & \quad \\{\quad{{\Delta\quad E_{driving}} = {{10301 - 2112} = {8189\quad{watts}}}}} & \quad\end{matrix}$For the reforming stream: $\begin{matrix}{{Output}\quad{Stream}} & {{Input}\quad{Streams}} \\{E_{{CH}\text{-}{reformate}} = 28320} & {E_{{CH}\text{-}{isooctane}} = 25442} \\{E_{{PH}\text{-}{reformate}}\underset{\_}{= 1235}} & {E_{{PH}\text{-}{isooctane}} = 0} \\{\quad 29555} & {E_{{CH}\text{-}{water}} = 1069} \\\quad & {E_{{PH}\text{-}{water}}\underset{\_}{= 0}} \\\quad & {\quad 26511} \\{{\Delta\quad E_{product}} = {{29555 - 26511} = {3044\quad{watts}}}} & \quad\end{matrix}$The oveall exergetic efficiency of the Compact Microchannel SteamReforming Unit is the change in exergy in the product stream (i.e., thesteam reforming stream) divided by the change in exergy in the drivingstream (i.e., the combustion stream), orε_(2nd Law) =ΔE _(product) /ΔE _(driving)=3044/8189=0.3717 or 37.2%For the calculation where moderate pressure drops were assumedthroughout the unit, the overall energetic efficiency dropped to as lowas 25%, due to additional exergy destruction through pressure drop(friction). These values compare to an overall energetic efficiency ofabout 30% for the synthesis gas production subsystem of an ammoniaproduction plant, as originally presented by Cremer (H. Cremer,“Thermodynamic Balance and Analysis of a Synthesis Gas and AmmoniaPlant,” in Thermodynamics: Second Law Analysis, ed. R. A. Gaggioli,American Chemical Society, Washington, D.C., p. 111, 1980) andre-presented in Szargut (1984).Consideration of Exergy Analysis for Ortho-Cascaded Mixer-Combustors andSteam Reformers

There is a desire to keep the highest temperature in the system lowenough so that stainless steel can be used. In the illustratedembodiment, this can be accomplished through the use of ortho-cascading.

In the current flowsheet, the highest temperature of any stream occurswith the combustion products that leave the Mixer-Combustor 1, at 725 C.Mixer-Combustor 1 also has the highest quantity of exergy destruction ofany unit in the system, 1600 watts. The literature confirms thatcombustion is often a large source of exergy destruction, and in thecase of the Mixer-Combustor 1, this is caused in part due to therelatively low temperature (400 C) of the air stream that enters thecomponent.

Leaving the steam reformer, the combustion stream has only been droppedto 662 C, therefore allowing Mixer-Combustor 2 to require only about ⅕as much fuel as was required for Mixer-Combustor 1. Mixer-Combustors 3and 4 receive the same benefit.

The end effect is this: When considering the subsystem consisting of theMixer-Combustor 1 and the Steam Reformer 1, the exergetic efficiency ofthis subsystem is only 22.3% (as can be calculated using the exergyvalues of Figure (b)). When adding Mixer-Combustors 2-4 and SteamReformers 2-4, via the ortho-cascaded route, the subsystem expands, withan upward improvement in the exergetic efficiency of the subsystem,increasing it to 52.8%. Thus an ortho-cascaded approach allows thesystem to simultaneously increase the exergetic efficiency of theoverall system while likewise allowing the system to be constructed fromrelatively inexpensive, stainless steel.

Consideration of Exergy Destruction and Exergetic Efficiency withMicrochannel Steam Reformers

FIG. 16 shows that, collectively, the Steam Reformers 1-4 destroy 852watts of exergy. Within each steam reformer, a portion of the physicalexergy of the combustion gas stream is initially transferred to thereforming stream, creating an increase in its physical exergy, then aportion of its physical exergy is transformed into chemical exergy, aspart of the catalytic steam reforming reaction. Generally, the exergeticefficiency of each steam reformer can be separately calculated basedupon the definition of exergetic efficiency, using the information thatis contained within FIG. 16. As an example, the exergetic efficiency ofSteam Reformer 1 is as follows:ε_(2nd Law)=(7080+845−6628−777)/(6648−5914)=520/734=0.708or about 70.8%. The other steam reformers likewise operate with similarexergetic efficiencies. Since the destruction of exergy, in this case,is internal to an individual component, improvements to the exergeticefficiency of these units would have to be realized through internaldesign improvements.Consideration of Exergy Destruction within the overall CompactMicrochannel Steam Reforming UnitThe cumulative exergy destruction of the system, totaling 5147 watts,can also be considered from FIG. 16. The principle contributors toexergy destruction within the unit are estimated to be themixer-combustors, the combustion recuperators, the water vaporizers, andthe steam reformers, with their collective exergy destruction being 4722watts, or 91.7% of the cumulative exergy destroyed. Mixer-Combustor 1provides the most exergy destruction, 1600 watts, or 31.1% of the totalexergy that is destroyed.

The Combustion Gas Recuperators have the second highest amount of exergydestruction. Air enters these units at 137 C, and is heated to 400 C bythe combustion gases, which drop from 654 C to 427 C while providing3974 watts (physical enthalpy from FIG. 17) to the air stream. Theseunits operates across a large temperature range (654−137=517 C), and haslarge terminal temperature differences (respectively, 427−137 at the lowtemperature end of the heat exchanger, and 654−400 at the hightemperature end), which contribute to a substantial amount of exergydestruction (901 watts). The Second Law efficiency, or exergeticefficiency, of this unit is calculated from the exergy values providedin FIG. 16 as follows:ε_(2nd Law)=(3013−1397)/(6169−3652)=0.642 or 64.2%

By comparison, the Reformate Recuperators present a high degree ofexergetic efficiency, with only 120 watts of exergy being destroyedwithin this unit. Here, steam and iso-octane enter the unit at 174 C,receiving heat from the reformate stream in the amount of 2870 watts(physical enthalpy), and increasing in temperature to 651 C. Thereformate stream cools from 700 C to 252 C in this transaction. Theseunits therefore also act across a large temperature difference(700-174=526 C), but have much smaller terminal temperature differencesthan with the Combustion Gas Recuperator. The exergetic efficiency ofthis unit is calculated to be:ε_(2nd Law)=(3107−1527)/(3380−1680)=0.929 or 92.9%

The Combustion Gas Recuperators and the Reformate Recuperators have eachbeen designed to be very compact heat exchangers, and each has beenfabricated to be integral with at least one other set of heatexchangers.

Why are the Combustion Gas Recuperators so much less exergeticallyefficient than the Reformate Recuperators? The answer lies in the designof the entire unit, which for this embodiment has the combustion gasstream providing the heat for vaporizing water for the reforming stream.As is shown in FIG. 16, vaporizing the water requires 4371 watts of heat(thermal enthalpy), and in order to accomplish this, the combustiongases must leave the Combustion Gas Recuperators at a sufficienttemperature (˜427 C) to provide this.

The Water Vaporizers likewise suffer from poor exergetic efficiencies,as an unavoidable consequence of the fact that water does not increasein temperature as it boils. The result is that the Water Vaporizerslikewise have a large terminal temperature difference at the hot end ofthe units (combustion gases enter at 427 C, whereas steam leaves at 160C), and a total of 598 watts of exergy is estimated to have beendestroyed in this unit.

In any thermo-chemical system, the exergetic efficiencies of variousunits are often not truly independent of each other. For the system athand, since the Combustion Gas Recuperator must be allowed to onlypartly make use of its exergy for preheating the air stream, theMixer-Combustor 1 must accomplish the rest of the air heatingrequirement, and it therefore requires substantially greater fuel thanthe other Mixer-Combustors (2-4) in the ortho-cascaded train (5538 wattsof chemical enthalpy for Mixer-Combustor 1 as opposed to 1137 watts ofchemical enthalpy for Mixer-Combustors 2-4 each). Thus, the need tovaporizer water has also accounted for a large degree of the exergy thatis destroyed within Mixer-Combustor 1.

What is apparent from this assessment is that, in the present embodimentof the Compact Microchannel Steam Reforming Unit, the sequence of WaterVaporizers, Combustion Gas Recuperators, and Mixer-Combustor 1 providesfor the greatest amount of exergy destruction (3099 watts, or 60.2% ofthe exergy destroyed). Accordingly, alternate system configurationsshould be explored that might improve the overall exergetic efficiencyof the system.

As a first consideration, the Exergy Band diagram of Figure (b) showsthat the physical exergy of the combustion gas stream is at 6169 wattsafter leaving Steam Reformer 4. In principle, this means that if thecombustion gas stream were reversibly brought into physical equilibriumwith the surrounding environment, it could provide up to 6169 watts ofwork (e.g., shaft work). This is a large value compared to the 10 kWe ofelectricity output that is desired from the fuel cell. While no systemis truly reversible, expansion units such as turbines and scrollexpanders are often exergetically efficient (80% or higher).

In order to accommodate this option, additional chemical enthalpy wouldneed to be added in the system, presumably at Mixer-Combustor 1 or priorto Mixer-Combustor 1. From Figure (a), it is observed that the chemicalenthalpy that would need to be added is 3974 watts (to replace theCombustion Gas Recuperators), 4371 watts (to replace the WaterVaporizers), and 1648 watts (to replace the Air Preheaters), for a totalof 9993 watts. Assuming that we were able to accomplish this, andassuming an 80% efficient expansion device, then the ratio of additionalwork to additional heat required is0.80×6169/9993=0.494, or 49.4%In actuality, an 80% efficient expansion device would result in theeffluent stream of combustion gases being hot enough to provide somepreheating of the combustion air, so the efficiency with which the newwork would be provided is somewhat higher.

Further observing that the physical exergy of the combustion gas streamdrops from 6169 watts to 3652 watts across the Combustion GasRecuperators, then to 1747 watts across the Water Vaporizers, otheroptions present themselves. For example, taking the same approach as inthe preceeding paragraphs, replacing only the Combustion GasRecuperators with an 80% efficient expansion device would provide0.80×(6169−3652) watts, or 2014 watts (shaft work). This is somewhatlower than in the previous alternative, but as the additional heat toreplace the Combustion Gas Recuperators is only 3974 watts, theincremental energy efficiency is0.80×(6169−3652)/3974=0.507, or 50.7%.This value is only slightly higher, but it avoids the problem of findingadditional heat for the water vaporizer.

Overall, the system could be revised in a number of ways, for betterexergetic efficiency, if a better source of heat can be found forvaporizing water. Exergetic considerations here provide anotheropportunity, which takes into account the observation that the CompactMicrochannel Steam Reforming Unit is designed for a system that includesa fuel cell plus other gas processing/conditioning hardware.

In particular, it is observed that a 10 kWe fuel cell will generatesubstantial waste heat. For example, a Proton-Exchange Membrane (PEM)fuel cell will operate at about 80 C, with about 60% efficiency (firstlaw). The result is that there is about 6000-7000 watts of heatavailable from the PEM fuel cell at about 80 C. The temperature of thisheat source is too low for it to provide for direct vaporization ofwater at 150-160 C, however, it can be upgraded via the use of a heatpump.

From a Second Law perspective, upgrading the heat from the fuel cellwill require the use of exergy. For a reversible heat pump, operatingbetween 80 C and 160 C, the Coefficient of Performance (COP) can becalculated to be:COP=(160+273)/(160−80)=5.41thus suggesting that the reversible heat pump needs 4371/5.41=808 wattsof exergy, or shaft work, in order to operate. A conservative assumptionwould put this at, say, 25% higher, or 808×1.25=1010 watts.

Using energy from the fuel cell to vaporizer water would allow redesignof the Combustion Gas Recuperators and the Mixer-Combustor 1, so thatthey operate with considerably better exergetic efficiency.Conservatively assuming that the redesigned Combustion Gas Recuperatorsonly destroy, say, 200 watts (as opposed to 120 watts of exergydestroyed in the Reformate Recuperators), and that the redesignedMixer-Combustor 1 likewise destroys, say, 400 watts (as opposed to 266,263 and 238 watts of destroyed exergy in Mixer-Combustors 2 through 4),a total of 600 watts of exergy would be destroyed. The resulting exergysavings is 1600+901+598−200−400=2499 watts.

In this case, we have gained 2499−1010=1489 watts of exergy, compared tothe 10 kWe output that otherwise was desired. A better design for theheat exchangers and combustors would likely cause this to increase. Withthis in mind, we could accordingly decrease the fuel consumption for a10 kWe output, by at least 1.489/10=0.1489 or about 15%.

CLOSURE

While preferred embodiments of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1-17. (canceled)
 18. A microchannel apparatus comprising: a header; atleast two flow microchannels, at least two orifices; wherein an orificeconnects said header with each flow microchannel; and wherein the ratioof the cross-sectional area of each of said at least two orifices to thecross-sectional area of the flow microchannels connected to saidorifices is between 0.0005 and 0.1.
 19. The microchannel apparatus ofclaim 18 wherein said apparatus comprises a water vaporizer.
 20. Amethod of vaporizing water in the microchannel apparatus of claim 18comprising the steps of: passing liquid water into the header; andvaporizing water in said microchannels. 21-23. (canceled)
 24. Amicrocomponent apparatus for conducting unit operations comprising: afirst microcomponent device having a first inlet, first exit, firstheader, a first array of microchannels and a second array ofmicrochannels; wherein the first inlet is connected to the first arrayof microchannels; wherein the first array of microchannels are connectedto a first exit that is connected to the first header; wherein, duringoperation, a first stream enters the first inlet of the firstmicrocomponent device and is distributed among said first array ofmicrochannels and a first unit operation is performed on said firststream, said first stream passes through the first exit into the firstheader; said first header being capable of modifying said first streamby a second unit operation; said first header connected to a secondarray of microchannels within the first device; wherein, duringoperation, said first stream enters said first microcomponent device andis distributed among the second array of microchannels wherein saidfirst unit operation is again performed on first stream. 25-48.(canceled)
 49. The microchannel apparatus of claim 18 wherein said flowmicrochannels have dimensions of: a height of about 100 to about 2500micrometers; a width of about 1.3 to about 13 millimeters; and a lengthof of about 1 to about 30 centimeters.